I posted

a video on youtube briefly summarising how to take shots of the curvature of the horizon, and hopefully it will inspire some people to do this. A few responses, however, have shown that some don't think it's possible, given that the general understanding is that the horizon is 'flat'.

As Mick and others have explained, that's true - and yet, there is also a curve. The simplest way I can think of to explain it is to imagine a tiny being hovering above a coin. The coin is a 'flat disk', yet the edge of the coin is curved, and can be photographed.

Compressing images from Google Earth provides theoretical support for this, as does Walter Bislin's round earth/flat earth simulator:

This shows a flat earth horizon on the left and globe earth horizon on the right, for a viewer at an elevation of 656 feet with a field of view of 60°.

Here's the vertically stretched flat earth horizon:

And here's the one for the globe:

Due to the resolution of the screenshot, it's not as smooth a curve as we would expect from a good quality shot of the actual horizon, but it's clearly there.